Heretofore, short haul lightwave transmission systems, i.e., those covering distances up to about 10 kilometers, have generally employed directly modulated LEDs in tandem with multimode optical fibers. Such multimode fibers typically have an attenuation of approximately 1-2 dB per kilometer and have relatively little bandwidth (0.5-1 GHz-km). In contrast, long haul light guide transmission systems, i.e., those covering distances of up to about 40 kilometers, employ directly modulated lasers driving single mode optical fibers. Single mode optical fibers typically have a core diameter of about 8 microns, an attenuation of about 0.5 dB per kilometer and nearly unlimited bandwidth (more than 1000 GHz-km). An LED or a laser is directly modulated when the drive current to the LED or laser is directly subjected to the modulating signal.
The single mode optical fiber information carrying capabilities cannot be fully realized when a relatively inexpensive LED serves as the optical power source, because LEDs provide incoherent and multimode emission which limits the achievable power coupling efficiency and limits the useful bandwidth of the link (due to dispersion). The single mode optical fiber works best when a relatively expensive laser operating in a single mode and with a narrow spectral width serves as the optical power source. For this reason relatively inexpensive LEDs are used in combination with relatively high loss and narrow bandwidth multimode fibers in short haul lightwave transmission systems. It has usually not been economical to use relatively expensive directly modulated lasers with single mode fibers in short haul lightwave transmission systems.
One short haul lightwave transmission system in which lasers are used in combination with single mode optical fibers is described in U.S. patent application Ser. No. 680,398 filed on behalf of S. D. Personick and S. S. Cheng and assigned to the assignee hereof. This patent application is incorporated herein by reference. In the system of the aforementioned patent application, all of the power of the laser is not dedicated to a single transmission path, as would follow from the use of a directly modulated laser. Instead, the power output of one or more unmodulated centrally located lasers is divided over a plurality of single mode optical fibers. External modulation is performed after power splitting. When a single laser can be used to supply optical power to a plurality of optical fibers, it begins to make economic sense to use relatively expensive lasers in combination with low loss single mode optical fibers in short haul optical transmission systems.
More particularly, U.S. patent application Ser. No. 680,398 discloses an optical transmission system for transmitting information between a first and a seoond central office. The optical transmission system includes n individual two way links between the first and second central offices. The two central offices are connected by two sets of n single mode optical fibers, one set for transmitting information from the first central office to the second central office and the other set for transmitting information from the second central office to the first central office. Each of the individual two way links includes one fiber from each of the two sets.
The first central office includes two lasers, one operating at wavelength .lambda..sub.1, and the other operating at wavelength .lambda..sub.2. The wavelength .lambda..sub.1 power output of the first laser is divided into n separate outputs by an n way power divider. The wavelength .lambda..sub.2 power output of the second laser is also divided into n separate outputs by an n way power divider. Each of the n wavelength .lambda..sub.1 outputs is then modulated with information and wavelength multiplexed with one of the n unmodulated wavelength .lambda..sub.2 outputs for transmission from the first central office to the second central office over one of the n optical fibers provided for this purpose.
At the second central office the multiplexed wavelengths are separated. Each of the n fibers used to transmit information from the first central office to the second central office has a demultiplexer associated with it for this purpose. The information modulated on the wavelength .lambda..sub.1 radiation is detected while the wavelength .lambda..sub.2 radiation is remotely modulated at the second central office. The remotely modulated wavelength .lambda..sub.2 radiation is transmitted back to the first central office over one of the second set of n optical fibers provided to transmit information from the second central office to the first central office. No optical power sources are required at the second central office.
Thus a total of n individual two way links are formed between the first central office and the second central office. To form the n links, the first central office requires two n-way power dividers, n wavelength multiplexers, and n modulators. Such a multiplicity of components is both expensive to purchase and difficult to assemble. Accordingly, it is the object of the present invention to develop a short haul optical transmission system using low loss single mode optical fibers in combination with the lasers, which system uses fewer components so that it is less expensive and easier to assemble than the system described above.